Nutritional value of mixed diets in cattle. A thesis. submitted in partial fulfilment. of the requirements for the degree

Transcription

1 LINCOLN UNIVERSITY LIBRARY CANTERBURY, NZ. I! Nutritional value of mixed diets in cattle A thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science in the University of Canterbury by Frederick Nii Ako Odoi Lincoln College 1985

2 ( i ) Abstract of a thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science. The nutritional value of mixed diets in cattle by F.N.A. Odoi Field trials suggest that the nutritive value of high quality, high moisture feeds may be improved by the addition of a small quantity of roughage. The nutritional value to steers of two high quality, high moisture feeds, fresh cut pasture and pressed beet pulp silage (PBPS), with and without various levels of straw was investigated. The general effects of straw substitution on the two diets and possible associative effect on OMO and MRT of OM, PEG and Cr-mordanted straw in the reticulo-rumen were measured in steers. Barley straw was fed with pasture at 0, 20, 50 and 100% of OM! in a 4x4 Latin Square design over an 80 day period, and with PBPS at 0, 50 and 100% of OM! in a 3x3 Latin Square design over 60 days to rumen fistulated steers. On two selected days, a single meal was offered to steers and from 0 to 12 hours after feeding total rumen contents were baled and weighed on four occasions. At 0 hr, PEG and Cr-mordanted fibre as liquid and solid phase markers were mixed with rumen contents. Rumen digesta was sub-sampled for OM, PEG and Cr analysis at each baling session. ~ vivo OM and OM digestibilities were measured from day 10 to 20. The general effect of adding 10% straw to pastur~ ~nd PBPS was to reduce OMO by 2% units and 1.8% units respectively. MRT of OM, PEG and Cr in the rumen increased by 1.6, 0.5 and 1.2% for pasture, and by 1.0, -1.2 and 1.0% for PBPS respectively for every 10% increase in straw

3 ! (i i) 1 e ve 1 i n diet. There was little associative effect on DMD of feeding barley straw with PBPS, but an increase of 1.8% units and 3.1% units over predicted values were measured with 20% and 50% straw DM fed with pasture. MRT for DM was 1.0 and 10.0% shorter at 20% and 50% straw with pasture respectively, and 6.5% shorter at 50% straw level with PBPS than would have been predicted from results of the diets fed separately. Similarly, shorter retention times for PEG, and Cr were observed when straw was fed with pasture or PBPS. A possible explanation for the positive associative effect on DMD of feeding straw with pasture is that pasture and/or straw digestibility was improved. Because straw DMD was likely to be close to its potential, it was calculated that pasture had to improve by 2.3% units on 20% straw and by 6.2% units on 50% straw fed with pasture to give the DMD values attained on the mixed diets. Had the improvement in digestibility of the mixed diets been due to an improvement in DMD of straw only, the DMD of straw would have to improve 9% units with the 20%, and 6.2% units with the 50% straw supplemented diets. Total OM mass of the rumen was higher at all times on the straw supplemented diets compared with the 100% pasture diet. This potential source of slow releasing energy in the rumen may allow better utilization of the high levels of ammonia in rumen fluid from pasture degradation. The trend towards shorter retention times of digesta in the rumen when straw is fed with pasture or PBPS is likely to pass soluble carbohydrates and N into the small intestines, possibly improving ME utilization, and voluntary intake of the diets.

4 ;, (i i i) ',_ I' CONTENTS Chapter 1 INTRODUCTION 2 LITERATURE REVIEW 2.1 Associative effects of feeds fed in combination. (1) Effect of a readily available carbohydrate source on roughage digestion. (2) Roughage supplementation of pasture. (3) Roughage supplementation of forage crops and green feed cereals. SUMMARY OF SECTION Effect of site of digestion on metabolizable energy util ization. (1) Disappearance of dig~sta from the reticula-rumen. (2) Factors that affect the disappearance of digesta from the reticula-rumen. (3) Measurement of retention time of digesta in the reticula-rumen. (4) Techniques for measuring retention time of digesta of digesta in the reticula-rumen. (5) Use of markers to measure passage of different fractions of di gestae SUMMARY OF SECTION Feeds (1) Pasture (2) Pressed beet pulp silage (3) Barley straw SUMMARY OF LITERATURE REVIEW 3 MATERIALS AND METHODS 3.1 Feeds 3.2 Experimental design and treatments 3.3 Measurements (1) Apparent coefficient of digestibility determination (2) Retention time determination 3.4 Laboratory analytical methods 3.5 Statistical analysis Page Z ,

5 ( i v) 4 5 RESULTS Chemical composition Mean DMI and apparent digestibility coefficients Mean retention times Total OM, fluid volume and ph of rumen digesta Concentration of ammonia, total PEG and Cr in rumen digesta Total OM, ammonia and ammonia:om ratio of rumen digesta 37 DISCUSSION Chemical composition of feeds General effects of including straw with the basal diet 52 (1) Digestibility of dry matter 52 (2) Mean retention time of dry matter 53 (3) Mean retention time of liquid phase of digesta 54 (4) Mean retention time of solid phase of digesta 54 (5) Comparison of liquid and solid phase turnover in the rumen 55 (6) Limitations of markers used 55 (7) Relationship between DMD and MRT-DM at different levels of straw inclusion in the diet (8) Total ammonia levels in rumen fluid 5.3 Complimentarity between the feeds offered together (1) Digestibility (2) Mean retention time 5.4 Recommendations for further research CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES APPENDICES

6 ( v) LIST OF TABLES Table 1 Solubility of dietary N in some ruminant feeds. 7 2 Approximate chemical composition (%OM) and energy value of pasture at different stages of growth. 7 3 A comparison of the chemical composition of pressed beet pulp and pressed beet pulp silage on dry matter basis Coefficients of digestibility (%) of constituents in pressed beet pulp Chemical composition of feeds (%OM) during trial periods Mean dry matter intake (kg/day) and mean coefficients of digestibility for dry and organic matter (%) of the diets Significance of associative effects on dry matter intake (kg/day) and digestibilities of dry and organic matter (%) of diets supplemented with barley straw Regression of mean dry and organic matter digestibilities (%) on the level of barley straw (%OMI) in the diet Regression of mean retention times (hours) of dry matter, PEG and Chromium on the level of barley straw (%OMI) in the diet Mean retention time (hours) and mean flow rate (%/hr) of digesta dry matter, PEG and Chromium from the reticulo-rumen for the different treatments A comparison of some retention time results (hours) from a review of the literature. 45

7 ( vi) 12 Significance of associative effects on mean retention time (hours) of dry and organic matter, PEG and chromium in the reticulo-rumen of diets supplemented with barley straw Mean total organic matter (kg), mean total ammonia-n (g) and the ammonia:om ratio (xl0-3 ) of reticulo-rumen contents soon after feeding (Ohr), and 4, 8 and 12 hours later Mean total dry matter (kg), mean fluid volume (1) and ph of reticulo-rumen contents soon after feeding (Ohr), and 4, 8 and 12 hours later. 15 Mean concentrations of ammonia-n (mg/l), total PEG (g) and total chromium (g) in reticulo-rumen contents soon after feeding (Ohr), and 4, 8 and 12 hours later

8 ( vi i) Figure 1 LIST OF FIGURES A stylized representation of ways in which release of ammonia-n and energy during fermentation can alter the efficiency of microbial utilization of degraded dietary protei n. 6 2 Model for disappearance of digesta from the reticula-rumen Time table of various operations in an experimental period The effect of level of straw (%DMI) on the mean apparent di gestibility of dry matter (%) in the rumen of steers fed mixed diets of pasture/beet silage and barley straw. 6 The effect of level of straw in the ration (%DMI) on the mean retention time (hours) of digesta dry matter in the reticula-rumen The effect of level of straw in the ration (%DMI) on the mean retention time (hours) of phases of the digesta (liquid and solid) in the reticula-rumen of steers fed mixed diets of pasture/beet silage and barley straw The relationship between apparent digestibility (%) and mean retention time (hours) of dry matter in the reticula-rumen of steers fed mixed diets of pasture/beet silage and barley straw The effect of level of straw in the diet (%DMI) on the ratio of total ammonia-n (g) to total organic matter (kg) in the reticula-rumen contents of steers fed a mixed diet of pasture and barley straw soon after feeding (Ohr), and after 4, 8 and 12 hours. 58

9 1 CHAPTER ONE INTRODUCTION Ruminants grazing high quality, high moisture pastures often grow at rates below that which would be predicted from feeds of such high nutritive value (Gibb and Treacher,1972; MacRae and Ulyatt,1974). Similar observations have been reported with certain forage crops such as swedes and turnips offered at high feed allowances (Drew, 1967; 1968) Farmer opinion is that when ruminants on such diets are given access to some roughage (even low quality hay or straw), their performance in terms of growth rate appears to improve. Experimental results have shown improved growth rate of both sheep and cattle when such high moisture feeds have been offered with some amount of roughage (Drew, 1967; 1968; Johnson, Arnold, Ellis, Smith, Lippke and Greene, 1983). The exact role of this small quantity of roughage in the utilization of these feeds is not clear. There have been suggestions that such low quality roughages, even at low intakes, might improve the nutritive value of high quality, high moisture feeds with which they are fed. If this was experimentally verified and explained, it would extend the use for such by-products which are at present under-utilized as a feed resource, and more importantly ensure a better utilization of such high quality feeds. The areas identified for investigation were: (i) Associative digestibility effects of a high and low digestibility feed fed in combination that may exist. (ii) A change in metabolizable energy utilization, possibly as a result of a shift in the sites of digestion of one or more component of the two feeds that may take place. (iii) A possible inefficient utilization of dietary protein of pasture may occur because of a high rate of protein degradation in the rumen, which is not matched by an adequate source of energy in young, succulent pastures but might be available with addition of the roughage.

10 2 (iv) A high level of ammonia in rumen fluid from degradation of pasture protein may correct the N deficiency in roughage, and improve its rate and extent of digestion. The experiments which form the basis of this thesis were undertaken to identify some of the causes of the observed effects of supplementing succulent, high quality feeds with small amounts of roughage.

11 3 CHAPTER TWO. M LITERATURE REVIEW The nutritive value of a feed and its components is commonly expressed as apparent digestibility. For a mixed ration, this is usually considered as the weighted average of the apparent digestibilities of the individual feed components of the ration. This approach is supported by trials that have shown apparent digestibility coefficients equal to the weighted means of the individual feed constituents making up the ration (Fenner and Barnes, 1966; Wainmann, Smith and Dewey, 1979; Byers, Johnson and Preston, 1982). For example, the work of Wainmann et ~ (1979) with steers fed at 1.5 times maintainance on mixtures of maize silage and barley showed, through complete energy balance trials, that ME values of the dietary components were addit i ve. The implication has been therefore, that there is usually little influence of one feed component on the digestibility of others with which it is combined, and thus values for the dietary components are additive, with no evidence of large associative effects Associative effects of feeds -- fed in combination Evidence from other trials (Schneider and Flatt, 1975; Byers, Johnson and Matsushima, 1976; Orskov, 1977) however, have shown that two feeds offered together are not necessarily additive in value, and the digestibility coefficient of the ration is not necessary a weighted average of values of the individual constituents determined separately or indirectly. One or more of the feed constituents can influence the digestibility of the others in the mixture. The effects of combining feeds may be either a direct enhancement or depression in digestibility, or indirect effects resulting from the correction of a dietary inbalance or from nutritive supplementation of one feed component by another (Schneider and Flatt, 1975). The occurance of associative effects between two feeds appear difficult to predict. A review of work on combined feeds of similar type to those used in the trials for this thesis shows the following

12 1 I 4 results. (1) Effect of ~ readily available carbohydrate source on roughage digestion. The rate of digestion of the cellulose portion of a roughage when it is fed with a grain based diet is reduced, and the roughage source is then poorly digested (Kane, Jacobson and Damewood, 1959; Colucci, Chase and van SO'est, 1982). The nutrient most affected is the crude fibre fraction, often with little decrease in digestibility of the other chemical constituents (Horton, 1979; Church and Santos, 1981). The reduced crude fibre digestion is due to a depression in cellulolytic bacterial activity. There is some indication however that crude fibre digestibility increases with rising level of straw (Forbes et ~, 1969; White et ~, 1975). In fattening steers on mixed rations of whole maize and maize silage, Byers et ~ (1976) reported that ME values for both items fed in combination were depressed when compared with either component fed on its own, indicating a negative associative effect on combination. With moderate levels of a soluble carbohydrate source however, the intake of roughage, as well as total dry and organic matter may increase which can, in terms of DE intake, compensate for the decrease in digestibility (Kane et ~,1959; Church, 1976). In an attempt to compensate for the dilution of ration energy with straw, animals tend to consume more feed (O'Donovan and Ghadaki, 1973). As the straw level is further increased (above about 30% of DMI), some data indicate that feed intake is reduced, and digestibilities of dry and organic matter, as well as energy are suppressed (Forbes, Raven and Irwin, 1969; White, Hembry and Reynolds, 1975). With a straw inclusion level of about 30% therefore, the higher total feed intake approximately compensates for any reduction in digestibility to give similar organic matter and digestible energy intakes as achieved with the 100% concentrate diet. Above 20-30% level however, digestible energy intake is depressed.

13 5 (2) Roughage supplementation of pasture Ruminants grazing lush pastures (high crude protein, and low structural fibre content) did not attain growth rates obtained with concentrate diets of similar digestibility (Nicol and McLean, 1970; Gibb and Treacher, 1972; MacRae and U1yatt, 1974). Recent work in New Zealand with sheep on diets of fresh rye-grass and white clover (U1yatt, MacRae, Clarke and Pearce, 1975), has shown that approximately 70% of the dietary protein is degraded by micro-organisms in the reticu10-rumen. The quantity of protein-n arriving at the duodenum was much less than the protein intake, with the difference (net N loss across the rumen) getting larger as dietary-n intake increased. Williams and Gordon (1983) demonstrated the presence of high levels of urea in the urine of sheep on such pastures and on high protein feeds such as lucerne hay. They showed that as much as 50% of the dietary-nintake on such diets was lost in the absence of a non-nitrogen source of energy for microbial use. MacRae and Reeds (1980) presented this graphically in FIGURE 1. The loss of ammonia-n to the animal in situation (b) in Figure 1, can be as high as 25% of the apparently digested-n of a feed such as fresh ryegrass (MacRae and Reeds, 1980). This raises the question of whether many fresh forages are being inefficiently utilized because of their high degradabi1ity in the rumen (TABLE 1). Poor microbial utilization of soluble plant protein should therefore be improved if enough easily digestible carbohydrates were made available. In a grazing trial where sheep were supplemented with barley grain, Joyce (1971) reported no effect of the barley on rumen ammonia and VFA concentrations. However, he obtained reduced 1iveweight gains in proportion to the decrease in digestib1e-n intake following successive additions of barley. Some other results using roughages as the energy supplement have been more encouraging. ' Hildebrandt (1958) proposed that ruminants on succulent pastures had problems in regulating rumen ph, and may therefore not be able to fully adapt to the diet until the content of structural fibre in the pasture increased with further vegetative development. Supplementing such pasture with a low protein roughage such as straw or mature hay, might improve the efficiency of utilization of such high quality

14 6 FIGURE 1: A stylized representation of ways in which the release of NH -N and energy during fermentation can alter the 3 efficiency of microbial utilization of 'degraded dietary protein'. ** (Source: MacRae and Reeds 1980) (a) (b) (c) NH3 absorption.. -. NH3 ~ absorption ", ",... e.g. chopped dried grass e.g. fresh grass e.g. roughage + casein, Feedi ng '- Feeding Feeding (a) Where the release of NH 3 -N ~atches available energy supplies, and micro-organisms are able to utilize most of it. (b) (c) Where there is rapid release of NH 3 -N, and large amounts get absorbed directly from the rumen and is lost to the micro-organisms. Where rapid release of NH 3 -N occurs in the presence of a basal roughage diet.

15 7 TABLE 1 Solubility of dietary nitrogen in some ruminant feeds ** Source MacRae (1976) mg/g DM - Total N Total Soluble N % Solubility Fresh rye-grass Fresh clover Freeze stored clover Dried grass Barley TABLE 2 Approximate chemical composition (% DM) and energy value of pasture at different stages of growth ** Source: Scott et al. (1980) Stage of growth DM CP CF Ash OMD DOM/lOOKgDM ME (MJ/KgDM) Short leafy Mixed length leafy Pre-flowering Flowering Dry, stalky

16 8 pastures probably through prevention of a low rumen ph. Cattle grazing wheat pastures, and given access to sorghum stubble were reported by Johnson et ~ (1983) to have higher average daily weight gain compared to other cattle with no access to roughage. They explained that stubble provided additional energy allowing for increased utilization of the high level of dietary-n available in the wheat pasture, and also prevented the likelihood of dietary-n being inefficiently utilized as a source of energy. Other work by Barry (1981), and Cruickshank, Poppi and Sykes (1985) has highlighted the large amounts of N lost across the rumen, and suggested that protein supply to the small intestines was limiting lamb growth rates on pasture. Barry (1981) for example, obtained increased growth rates on infusing amino acid supplements into the abomasum of sheep, showing that ruminants fed fresh herbages of high digestibility and protein content, may nevertheless suffer from a deficiency in one or more essential amino acid absorbed from the small intestines. (3) Roughage supplementation of forage crops and green feed cereals Drew (1967; 1968) fed hoggets on swedes in two trials, and reported weight gains nearly 50% higher for animals allowed a small quantity of hay (350g/head/day) as supplement, compared with others on swedes only. This was in spite of the fact that the supplemented group consumed only 5% more digestible energy than the all swede group. The quality of hay fed did not appear very important, as results were similar for average and high quality hay. Differences observed between groups were attributed to a possible change in the different proportions of VFA's produced. Results obtained showed a similar trend in both indoor and outdoor work. Bines and Davey (1978) using pelleted diets, concluded that 20% chopped straw in the ration yielded the highest blood concentration of acetate, propionate and 6-hydroxybutyrate, nearly twice the amounts recorded in the absence of the roughage. They suggested that an inhibition of acetate absorption from the rumen on an all concentrate diet may lead to excessive accumulation at this site leading to an unusually low voluntary intake. However, the results obtained in Drew's work (1967; 1968) where digestible energy intakes were similar with ;

17 9 both roughage supplemented and unsupplemented groups, would question a low voluntary intake (resulting from acetate accumulation) and thus differences in VFA proportions, as the sole reason for the better growth performance recorded by the supplemented group. Another explanation for differences between the groups was that the extra heat of fermentation generated by the roughage reduced the energy cost to the hoggets of warming the ingested feed in winter, though this appears improbable in the light of similar results obtained indoors. Summary of Section 2.1 Results from several trials in which two or more feeds have been fed in combination suggest little effect of one on the digestibility and utilization of another. Other trials however, provide definite evidence for the existence of associative effects (both positive and negative) between two or more feeds. The reasons suggested in explanation of the complimentary effects include: (i) The different proportions of VFA's produced with two or more feeds offered together improves the efficiency of nutrient utilization. other. (ii) One feed supplies a nutrient that may be deficient in the (iii) A supplemented roughage serves as an additional source of energy, and thus improves the ability of the microbial population to utilize soluble dietary-n, and therefore reduces losses of ammonia-n across the rumen. (iv) The additional heat of fermentation generated, particularly with roughages, has an energy sparing effect in the maintenance of body temperature under cold conditions Effect of site of digestion on Metabolizable Energy utilization Some of the possible explanations for complimentarity between feeds involve a change in site of digestion.

18 10 The digestive system of the ruminant is unique in its combination of exogenous with endogenous enymatic hydrolysis. Microbial fermentation in the rumen provides microbial enzymes for hydrolysing e-glucose polymers (or fibre) and uronic acid esters, for which the host lacks metabolic pathways and, in the absence of which, these feed components are poorly utilized (Putnam and Davis, 1965). The end products of this fermentation are available for metabolism by the ruminant.. ~ Passage of di gesta out of the rumen results in the "fl ushi ng" out of micro-organisms and rumen indigestible material to the abomasum and small intestines (Owens and Isaacson, 1977), where the microbes are exposed to hydrolytic processes and provid~ essential amino acids for use by the host animal (Loosli, Williams, Thomas, Ferris and Maynard 1949) Most solids that leave the rumen are poorly digested in the intestines of ruminants. For those nutrients that can be digested enzymatically in the small intestines, ruminal fermentation is an inefficient process (Bull, Rumpler, Sweeney and Zinn, 1979). The site of feed digestion is therefore important in the efficiency of feed utilization. Some control over the site of feed digestion is provided by rate at which digesta disappears from the reticulo-rumen. (I) Disappearance of digesta from the reticulo-rumen Ingested feed leaves the reticulo-rumen basically by two routes: (i) By digestion and absorption, and (ii) By feed particles being broken down to a size small enough to pass through the reticulo-omasal orifice (Balch and Campling 1962). Waldo and Smith (1972) proposed a model for fibre disappearance from the reticulo-rumen based on what they termed 'potentially digestible and indigestible fractions' of the feed (FIGURE 2.). Wilkins (1969) described potential digestibility as 'the maximum digestibility attainable when the conditions and the duration of fermentation are not limiting factors'.

19 11 FIGURE 2: Model for di sappearance of.di gesta fr.om the reticula-rumen. ** Source: Waldo and Smith (1972) A Kl ~, POTENTIALLY PASSAGE DIGESTIBLE K2..., FRACTION PASSAGE B INDIGESTIBLE K2 FRACTION "-, PASSAGE (A) An indigestible fraction which disappears by passage only. (B) A potentially digestible fraction which disappears both by passage and digestion. = rate of digestion = rate of passage

20 12 Model for fibre disappearance from the rumen. The proposal of Waldo and Smith (1972) is that not all fibre present in feed of ruminants is potentially digestible, and that the digestion of lignified fibre proceeds as if the cellulose component is of two definable fractions: (A) an indigestible fraction which disappears from the reticulo-rumen by passage only (through the reticulo-omasal orifice), and (B) a potentially digestible fraction which disappears by both passage and digestion. The proportion of the potentially digestible fraction actually digested in the reticulo-rumen is represented by the ratio K1 / K1 + K2, where K1 = digestion rate, and K2 = passage rate Anythi ng that affects di gesta di sappearance from the ret i cu 1 o-rumen wi 11 therefore invariably influence the site of feed digestion. (2) Factors that affect the disappearance of digesta from the rumen. Several factors are involved in the disappearance of particulate matter from the reticulo-rumen. Most of these exert their influence through altering the length of time feed is retained in the rumen, and hence influence digestibility and intake (Balch and Campling, 1965). Of major importance are: (i) Level of feeding Tyrell and Moe (1975) stated that the amount of a ration consumed has a large influence on digestibility, and there is usually a depression in apparent digestibility with increasing level of feeding (Andersen, Reid, Andersen and Stroud, 1959; Lindgren, 1981; Colucci et ~,1982). The greatest reduction in retention time in the rumen is likely to be because of an increased feed consumption (Thornton and Minson, 1973), as digestibility of the microbial dependent fraction is likely to be reduced. disadvantage digestibility of the microbial dependent fractions.

21 13 (ii) Feed processing Processing methods such as grinding and pelleting usually cause a depression in digestibility (Balch, 1960; Minson, 1963), and an increase in intake and rate of passage from the rumen (Campling, Freer and Balch, 1963; Campling and Freer, 1966), provided adequate protein is present. Processing affects physical form, particle size and specific gravity, and thus the inherent rate of digestion (Campling and Freer,1960; Cole, Johnson, Owens and Males, 1976; Prange, Stern, Rode, Santos, Jorgenson and Satter, 1979). (iii) Rate of flow of water Since water is the medium in which dry matter in the rumen is dispersed, factors that affect the flow of water through the rumen will influence flow of dry matter from the rumen. Sutherland (1975) concluded that th~~low of water depended on the level of intake, rumen tactile stimulation, rumen motility, salivary flow and negative feedback from the rest of the digestive tract. Because 80-85% of fresh herbage is water, the retention times of water and organic matter in the rumen on such diets are low compared with other diets of similar digestibility (Ulyatt and MacRae, 1974). (iv) Dietary factors and rumen fill Stemmy material is retained longer than leafy material, and on the whole legumes have a shorter retention time when compared with grasses (Poppi, 1981). Cell contents are rapidly digested, and are generally absent from the rumen within six hours of ingestion (Hungate, 1966). Cell wall components on the other hand, often show a time lag before the onset of a slow fermentation and feed breakdown, and normally remain in the rumen for at least 24 hours (Mertens, 1977). Gut fill on anyone diet is relatively constant under ad lib feeding conditions (Minson, 1966). If rumen fill is increased with no corresponding increase in digesta disappearance, then intake will not change, but retention time and digestive efficiency will increase. It is usual however, for an increase in the passage component to be associated with an increase in gut fill (Tulloh, 1966).

22 14 (v) Physiological state Various physiological conditions of the animal may change the level of fill, rate of rumen digesta disappearance and intake. These include lactation, compensatory gain and cold exposure which increase rumen motility and thus the rate of passage, and depress digestibility (Tulloh, 1966; Kennedy, Christopherson and Milligan, 1976). (vi) Microbial growth Some chemical factors in the feed may affect bacterial growth, and the microbial speci,es present. Low rumen ammonia levels (Nolan, 1974), and deficiencies of sulphur (Bray and Till, 1974) reduce microbial growth, and the resultant rate of digestion may be less than is possible given the chemical moiety of the cell wall and its physical structure. Frbm this review of the sites of feed digestion in the ruminant, and the factors that have an influence on it, it would appear that retention time in the rumen of soluble, high moisture feeds will be relatively short. Adding a roughage to such diets should lengthen the time digesta is retained in the rumen. This may have an effect on digestibility and voluntary intake. (3) Measurement of retention time of digesta ~ the reticulo-rumen. From the above discussion, the concept of retention time of feed in the reticulo-rumen is important as it can give an indication of the likely major site(s) of digestion, and thus the efficiency with which the feed can be utilized by the host animal. Retention time in the rumen has been defined as 'the average time di gesta stays in the rumen I (Mi nson, 1966), or as 'the time requi red for the input of a component to equal that in the rumen I (Bull et ~ 1979; Minson, 1966). Mathematical relationships ~ retention time measurements Because of the differential rates of digestion of various components of the feed, measurements of feed retention time can be done either for whole digesta ego dry matter, or for specific components such as the liquid and particulate phases.

23 15 Retention time is related mathematically to some of the other parameters used in marker dilution studies. Several of these mathematical relationships were originally proposed by Hyden (1961). Variations of them have been used by Ulyatt (1964), Reid (1965) and Faichney (1975). For example, Retention time (hours) = 1 / K = / TO.5 where, K (refered to in the literature severally as, dilution rate or rate constant) is the fractional outflow rate, the proportion of rumen pool passing out in unit time. K is derived from the slope of the semi-log plot of the amount of material remaining in the rumen, against time. TO.5 is the the half life, or the time taken for half the pool size to disappear, and is a constant. Other parameters like rumen volume, flow rate, and rate constant can be calculated from the above relationship (Hyden, 1961; Ulyatt, 1964) Retention time measurements refer to disappearance by both passage and digestion. Owing to the flow of saliva into the rumen and the passage of feed components across the rumen wall, estimates of retention time are apparent and not absolute values for the feed involved, except for fibre (Minson, 1966). (4) Techniques for measuring retention time of digesta ~ the rumen The techniques commonly used in retention time measurements on digesta in the reticulo-rumen are: (i) Serial emptying (Reid, 1965) A single meal is fed each day, and the rumen is emptied at pre-determined time intervals over a 24 hour period. The inverse of the slope of the semi-log plot of the amount of any component remaining in the rumen against time, gives retention time for that component. The disadvantages of the technique are that a single meal may be abnormal, compared with the normal eating pattern and rumen physical functions may be impaired. Also the rumen is emptied several times in a 24 hour period, which might disturb anaerobic rumen function.

24 16 (ii) Steady state (Minson, 1966): With this procedure, the animal is fed at hourly intervals, and the rumen is emptied only once, midway between two feeds. This approaches a more normal feeding pattern, and the rumen need only be emptied once. It is therefore probably the better of the two techniques, although the serial emptying method also gives good results where steady state conditions cannot be imposed. Because water soluble constituents of the digesta pass out of the rumen more quickly than the insoluble material both during and after feeding, the breakdown of the latter will be incomplete before the ingestion of the next meal. There will therefore be in the rumen at any one time, the residues of several meals at various stages of digestion. Other problems that might be encountered with retention time measurements are : (i) Changes in concentration of a metabolite can be a result of differences in rates of production, absorption and passage, as well as in the volume of material in which the metabolite is dispersed (Reid, 1965) (ii) A layering of particle dry matter in the rumen may lead to an uneven distribution of feed components, making it difficult to obtain truly representative samples in vivo, particularly with cattle (Bryant, 1964) Markers used to follow the flow of individual fractions of the digesta, and total emptying of rumen contents prior to sampling, have helped overcome some of these problems. (5) Use of markers to measure passage of different fractions of digesta. A solution to the differential turnover rates of various fractions of the digesta has been the use of two markers to label the liquid and particulate phases separately. To be of value, the selected marker should be: (i) Strictly not absorbed in the gastro-intestinal tract

25 17 (ii) Unaffected by conditions in the tract, and in turn not affect the microbial population (iii) Physically similar to, and associated intimately with the fraction it is to label (iv) Capable of accurate estimation in digesta samples, and not interfere with other chemical analysis (Faichney, 1975). It should be noted however, that no one marker is likely to fully satisfy everyone of the above requirements. Liquid phase markers. Liquid phase markers are diluted and pass out in the flow of water and other fluids continuously leaving the rumen. They give an indication of how quickly soluble feed material not absorbed from the rumen, and the. portion of the di gesta dry matter which has been reduced to a small enough particle size, leave the rumen. Some of the more commonly used liquid phase markers are polyethylene glycol (PEG), and chromium ethylenediamine tetraacetate (Cr-EDTA). PEG ~ ~ 1 i qui d phase marker. PEG (MW 3350) appears to satisfy most of the criteria required of a good water soluble marker in the gastro-intestinal tract. Its distribution volume. has been shown (Uden, Colucci and van Soest, 1980) to be about 95% of total rumen water, although under some circumstances it has been known to associate with digesta dry matter to a small extent. Czerkowski and Brackenbridge (1969) showed it to be excluded from a small proportion of the water in beet pulp, and it is precipitated when given with feeds rich in tannin (Kay, 1969). It can be accurately estimated turbidimetrically, if care is taken with clarifying rumen liquor (Ulyatt, 1964; Malawer and Powell, 1967).

26 18 Particulate phase markers. Insoluble or particulate phase markers have a disappearance rate which depends on the specific gravity and particle size of the fraction they mark (King and Moore, 1957; Balch, Campling and Freer, 1961). They remain in the rumen until the solid fraction is reduced to fragments small enough to pass out through the reticulo-omasal orifice. I These particulate markers can be (i) Indigestible feed substances such as lignin and silica. (ii) Chemical elements firmly bound to fibre by mordanting ego chromium, cobalt and cerium. (iii) Loosely attached material to the digesta ego chromic oxide, stained feed particles or even plastics (Kotb and Luckey, 1972; Church, 1975; Uden et ~ (1980). Chromium mordanted straw fibre ~ ~ particulate marker Metal complexes with plant fibre should be good markers provided the bonding can withstand conditions in the gastro-intestinal tract, and the metal element is easily recovered from the fibre matrix to which it is bound. Chromium has been shown to form a stable complex with straw fibre (Uden et ~, 1980), making it suitable for use in retention time studies (Pienaar, Roux and van Zyl, 1983). The mordanting process is described in Appendix Method 1. Both liquid and particulate phase markers can be used with steady state and serial emptying techniques. Summary of sect ion 2.2. Retention time of ingested feed in the digestive tract is important in ruminants because of the role of the exogenous microbial population in rumen fermentation. Feeds, particularly those high in fibre if retained for long periods in the rumen allow time for maximum microbial breakdown, and the potential digestibility of the feed is more likely to be attained~ On the other hand, feeds capable of being digested in the small intestines would benefit from a short retention time in the rumen so that extent of rumen degradation is reduced, and the efficiency of utilization increased by more digestion in the small

27 19 intestines. Factors such as the level of feeding, the fibre content of the diet, and the animal's physiological state influence retention time of digesta in the gastro-intestinal tract, and are therefore important in efficient feed utilization. Because of observed differences in turnover rates of various fractions of the digesta, two or more markers are often used to measure retention times of different fractions. Markers have been used with either the serial emptying, or steady state technique. The latter may more closely resemble normal conditions in the grazing animal. Where facilities preclude the use of steady state methods, serial emptying has been shown to gi ve satisfactory results. 2.3 Feeds Two high moisture feeds were selected for feeding with roughage for digestibility and retention time studies. These were: (i) High quality succulent pasture (mainly ryegrass-white clover swards), which form the basis of grazing in New Zealand and many other temperate countries. (ii) Silage made from pressed sugar beet pulp (PBPS), a high energy by-product feed for livestock. (iii) Barley straw was selected as a readily available cheap roughage source to be fed with the high moisture feeds. (1) Pasture The dominant species'in the swards used for the trial were rye-grass (Lolium spp.) and white clover ( Trifolium spp.). Composition and nutritive value Table 2 shows the range in nutritive quality of pasture swards over the year. Young succulent pasture has a high crude protein (20-35%) and soluble carbohydrates (10-20% of OM)content, but is low in structural carbohydrates (15-30% of OM). It has an organic matter digestibility of between 75-85% (MacRae, 1976).

28 20 The extent of microbial degradation of the dietary protein of pasture in the rumen is high with fresh young pasture (Table 1). Ammonia levels in rumen fluid will therefore usually exceed the capacity of micro-organisms to utilize ammonia for synthesis, and excess ammonia is absorbed across the rumen wall and excreted as urea in urine. The amount of N passing out of the rumen will therefore be considerably less than that ingested, limiting amino acid absorption in the ruminant (Barry 1981). The main determinants of the ability of micro-organisms to utilize dietary N are how degradable the protein is, and the availability of a source of energy in the rumen at the time of the release of ammonia. Given most diets, about 30g of microbial protein can be synthesized per kg OM fermented (ARC 1980). Beyond this, ammonia-n of any origin is nutritionally useless to the ruminant. Roughage supplementation of pasture Be~ause of the low level of structural carbohydrates in young pasture, a source of energy required by the rumen micro-organisms in order to be able to effectively utilize the high level of ammonia-n released can be limiting. It is suggested that the feeding of a roughage such as barley straw with pasture may provide additional slow release energy and therefore may be beneficial to the ruminant on such di ets (MacRae and Reeds, 1980; Johnson ~~, 1983). (2) Pressed Beet ~ Silage. In the manufacture of sugar or methanol from beet, the residue after extraction of the product is refered to as beet pulp. It is often pressed by rollers to remove excess water, leaving a by-product of about 20% OM. Because of the high moisture content, pressed beet pulp goes mouldy on exposure to air and loses its palatability and acceptance to livestock. It is therefore often preserved by ensiling in air-tight containers (Fairburn 1974; AOAS 1976), and refered to as pressed beet pulp silage (PBPS). Composition and nutritive value. When well preserved PBPS has a nutritive value comparable to the fresh material (Harland 1981). Table 3 compares the chemical composition of the fresh and the ensiled product.

29 21 TABLE 3 A comparison of the chemical composition of pressed beet pulp silage (PBPS) on a % Dry Matter basis ** Source: Kelly 1983 Constituent PBP PBPS Dry Matter Crude Protein Crude Fibre Ether Extract Ash Nitrogen free extractive (NFE) Neutral Detergent Fibre (NDF) Acid Detergent Fibre (ADF) Acid Detergent Lignin (ADL) Cellulose Gross Energy (MJ/KgDM) 17.1 Metabolizable Energy (MJ/KgDM) 12.1 Water soluble carbohydrates 4.0 Phosphorus 0.11 Calcium 0.95 Magnesium 0.19 Sodium 0.51 Potassium 0.78

30 22 TABLE 4 Coefficients of digestibility (%) of constituents in Pressed Beet Pulp (** Source : Sheehan and Quirke 1982) Constituent % Digestibility Dry!-1atter 81.1 Organic matter 85.2 Crude Protein 65.5 Crude Fibre 90.8 Ether extract 73.7 Acid detergent fibre (ADF) Neutral detergent fibre (NDF) 79.9 Lignin 69.2

31 23 Pressed beet pulp has a high ME value of between MJ/KgOM, comparable to a high energy grain such as barley (Harland, 1981; Kelly, 1983). The high value of pressed beet pulp as a feedstuff lies mainly in its high energy content, derived largely from a high fibre fraction made up mainly of cellulose and hemicellulose easily digested by ruminants. Because of its small particle size and non-rigid physical form it produces lower rumination times (78 minutes) compared with good quality hay (516 minutes) during a 24 hour period (Welch and S mi t h, 1971). It is not a good source of protein, containing only between 60 and 70g OCP per kgom. It has been suggested however that this protein might be largely undegraded in the rumen, due to heat treatment during the sugar extraction process. This protection could enhance the feeding value of the dietary protein of pressed beet pulp or silage made from it (AOAS 1976). PBPS is low in phosphorus, and some form of dietary mineral supplement is usually required if it forms a considerable portion of the ration for any length of time. TABLE 4 shows digestibility coefficients for the chemical constituents of beet pulp. The high digestibility of the fibre fraction, and results from production trials (Sheehan and Quirke, 1982) suggest that the ME value quoted for PBPS may be an underestimation of its true feeding value. Roughage supplementation of PBPS PBPS like young pasture has a high moisture content and is highly digestible. Unlike pasture however, it is high in structural carbohydrates although this is mostly digestible material. Its small particle size and high moisture content should promote a fast rate of passage through the digestive tract, and reduce overall digestibility of OM. Addition of barley straw to PBPS it is proposed would slow the rate of passage, and allow a high level of the potential digestibility of the fibre fraction to be attained.

32 24 Rumen 2!! on feeds high.i!!. soluble carbohydrates.. The end products of carbohydrate metabolism in the rumen are VFA's, which although continuously absorbed require the buffering effect of alkaline saliva to prevent a marked fall in rumen ph (Church 1975). Due to the high content of readily soluble carbohydrates in both fresh pasture and PBPS, there is the possibility of lactic acid accumulation leading to an acidic rumen environment (Briggs, Hogan and Reid, 1957). This will be detrimental to cellulolysis, which is inhibited below ph 6.1 (Mould, Orskov and Mann,1983). Many of the other extracellular rumen micro-organisms function within fairly narrow ph ranges (Church 1975). On adding straw to such feeds, there is usually a depression in DMD. This is due both to the lowered rumen ph and an increase in the rate of solubilization of the more readily degradable substrates. This latter effect cannot be corrected by ph manipulation as for the former because of the preferential. attack by micro-organisms on the readily fermentable substrates rather than the relatively complex fibre fraction. More frequent feeding of the ration helps stabilize rumen ph at peak value associated with that feeding regime (Kaufmann, Hagemeister and Dirksen, 1980). (3) Barley straw. Barley straw together with other cereal straws represent a vast potential feed resource world wide. Composition and nutritive value. Crude protein (1-5% of DM) and phosphorus have been identified by Anderson (1978) as the nutrients most usually deficient in straws. Their content of other minerals like Ca, Zn, Mg, Mn and Se may limit I animal productivity when straw is fed as the sole diet for an extended period (Mathison, Hardin and Beck, 1981). Straws are however high in structural carbohydrates and cell wall fraction may often account for 70-80% of plant DM (Jackson 1977). The cellulose and hemicellulose fractions are digestible to a large extent but their digestibilities are often reduced through lignification and interference from the silica fraction.

33 25 The slow rate of breakdown of straw increases appreciably its retention time in the rumen. This leads to low voluntary intakes (1-2% of liveweight) and therefore inadequate energy consumption (Balch and Campling, 1962; O'Donovan 1983). The main cause of the low voluntary intakes on straw is that CP levels are inadequate to promote efficient microbial growth. It is usually below the 8.5% threshold level quoted by Blaxter and Wilson (1963) as being necessary to prevent the inhibition of cellulolysis. Thus although potentially a good source of energy, DMD of most straws are only about 40% (Jackson 1977; O'Donovan 1983), which limit its use as a feed. Increased rate of passage and intake of straws has been achieved through supplementation, with large responses to extra protein reported by O'Donovan, Silva and Euclides, (1983). Energy supplements are less effective in the absence of protein. Feeding straw with pasture The effects of feeding straw with pasture should be mutually beneficial. The high level of ammonia-n released into the rumen from pasture should help to partially make up the N deficiency in straw. Straw on the other hand, would provide an extra source of energy for microbial growth and synthesis, allowing for the more efficient utilization of ammonia-n in the rumen. An optimum inclusion level of about 20% in mixed rations has been suggested from growth rate studies by Davendra (1975), and Davendra and Raghavan, (1978). Summary of literature review When two or more feeds are fed together, several results from the literature suggest little effect of one feed on the digestibility and utilization of the others. There is however evidence for the occurance of both positive and negative associative effects in some experiments where mixed diets have been fed to ruminants. Associative effects on digestibility of feeding mixed diets have been attributed to one feed correcting a nutrient deficiency in another with which it is fed, or the mixed diet providing a better balance of nitrogen and energy, so that ME utilization is improved. Associate

34 26 effects of feeding a mixed diet may also act by altering the rate of passage of ingested feed through the digestive tract. The time digesta is retained in the reticula-rumen is important because feeds high in fibre are better digested in the rumen (through the action of extracellular microbial enzymes) and benefit from a long retention time. High quality feeds capable of being digested in the intestines by host enzymes on the other hand, are more efficiently utilized when retained for a short time in the rumen. Ruminants grazing certain high quality feeds such as young pasture or forage crops grow at rates below what may be expected from the nutritive value of the feed. Feeding such diets with some roughage seem to improve growth rates. Reasons for this roughage effect have not been determined. This trial was undertaken therefore, to investigate any effects of feeding a high quality diet such as young pasture or beet pulp silage with some amount of straw on the digestibility, and retention time of digesta i~ the reticulo-rumen.